Antigen mediated histamine release form RBL-2H3 cells is associated with substantial hydrolysis of membrane inositol phospholipids and an increase in cytosol Ca2+ (CA SIGNAL). As reported last year, studies with variants of the RBL-2H3 cells revealed several variants that were completely unresponsive to antigen but did release histamine when challenged with a combination of ionophore and phorbol ester. The possibility of defective coupling between receptor aggregation and activation of phospholipase C was investigated. One such variant BUDR-2B1, showed no or very little phosphoinositide hydrolysis in response to stimulants of GTP- regulatory proteins such as sodium fluoride in intact cells and GTPgS in permeabilized cells. The isolation, purification and hybridization of mRNA with specific radiolabeled probes for two GTp-regulatory proteins, Gi and Go, revealed that the variant and the parental 2H3-cells transcribed message for Go but not for Gi. Another interesting variant showed substantial secretion of histamine and hydrolysis of inositol phospholipids but no Ca2+ signal as determined with quin 2, Fura 2 and uptake of 45Ca2+. However, the responses were still dependent on external Ca2+ and studies with 45Ca2+ revealed that intracellular bound Ca2+ was released without significant perturbation of free cytosol Ca2+ levels.